The process of drilling subterranean wells to recover oil and gas from reservoirs includes boring a hole in the earth down to the petroleum accumulation and installing pipe from the reservoir to the surface. Casing is a protective pipe liner within the wellbore that is cemented in place to ensure a pressure-tight connection to the oil and gas reservoir. The casing is run in continuous strings of joints that are connected together as the string is extended into the wellbore.
On occasion, the casing becomes stuck, preventing it from being lowered further into the wellbore. When this occurs, load or weight is added to the casing string to force the casing into the wellbore, or drilling fluid is circulated down the inside diameter of the casing and out of the casing into the annulus in order to free the casing from the wellbore. To accomplish this, special rigging is typically installed to add axial load to the casing string or to facilitate circulating the drilling fluid.
Further, when running casing, drilling fluid is added into each section of casing as it is run into the well. This fluid prevents the casing from collapsing due to high pressures within the wellbore acting on the outside of the casing. The drilling fluid also acts as a lubricant, facilitating lowering the casing within the wellbore. As each joint of casing is added to the string, drilling fluid is displaced from the wellbore.
The normal sequence for running casing involves suspending the casing from a top drive, or drilling hook on a rotary rig, lowering the casing string into the wellbore, and filling each joint of casing with drilling fluid. The filling of each joint or stand of casing as it is run into the hole is referred to as the fill-up process. Lowering the casing into the wellbore is facilitated by alternately engaging and disengaging elevator slips and spider slips with the casing string in a stepwise fashion, allowing the connection of additional joints or stands of casing to the top of the casing string as it is run into the wellbore.
Circulation of the fluid is sometimes utilized when resistance is encountered as the casing is lowered into the wellbore, preventing the running of the casing string into the hole. This resistance to running the casing into the hole may be due to such factors as drill cuttings or mud cake being trapped within the annulus between the wellbore and the outside diameter of the casing, or caving of the wellbore among other factors. To free the casing, fluid is pumped down through the interior of the casing string and out from the bottom, then through the annulus and up to the surface to free/remove any obstruction. To circulate the drilling fluid, the top of the casing is sealed so that the casing can be pressurized with drilling fluid. Generally, the fluid connection between the rig's mud pumping system and the interior of the casing string includes the rig's top drive and the casing fill-up and circulation tool. The casing fill-up and circulation tool typically includes a mud valve that selectively permits pumping of fluid (drilling mud) from the rig's mud system to the interior of the casing string. The casing fill-up and circulation tool also includes a seal assembly to seal the annular space between the interior of the casing and the outer diameter of the casing fill-up and circulation tool. Since the casing interior is under pressure, the integrity of the seal is critical to safe operation, and to minimize the loss of expensive drilling fluid. Once the obstruction is removed, the casing may be run into the hole as before.
Once the casing string has been assembled to the required length, a crossover connection may then be connected to the top of the last casing joint or string hanger. High strength drill pipe is then connected to this crossover connection. As this high strength drill string, known as a landing string, is assembled, the casing string is then lowered into its desired location within the wellbore.
A drill pipe flowback tool is used when lowering the landing string to allow drilling fluid that is expelled through the ID of the landing string to be contained and directed to a low back pressure port or to the top drive where it is directed back to a reservoir. Generally, the drill pipe flowback tools require the rig down of the casing fill-up and circulation tool in order for the drill pipe flowback tool to be rigged up to the rig's top drive.